Children with Attention Deficit Hyperactivity Disorder (ADHD) have numerous areas of neuropsychological dysfunction including response inhibition, working memory, and attention. One neuropsychological outcome measure that consistently reveals between-group differences is response variability. However, until recently, differences in response variability have been reported as an ancillary finding or viewed as a nuisance in the analyses. The specific aims of the present study are to 1) Examine response variability in ADHD patients across neuropsychological tasks to understand the breadth of this specific deficit and to understand the relation between response variability and other neuropsychological outcome measures;2) Assess whether response variability deficits are specific to either or both of the two most prevalent ADHD subtypes (i.e., Combined Type [CT] and Predominantly Inattentive Type [PIT]);3) Determine whether response variability in ADHD patients is affected by either medication or a variety of environmental manipulations (e.g., reward);and 4) Understand the relationship between neuropsychological measures of response variability and naturalistic instances of variable performance. Forty-five children (aged 7-11) with ADHD-CT, 45 children with ADHD-PIT, and 45 normal controls will be recruited to examine response variability across a wide range of neuropsychological tests. Task parameters such as event rate, stimulus saliency, and the presence of operant reward will be modified on each test to determine the conditions under which response variability is manifested in children with ADHD. In addition, all children with ADHD will participate in a placebo-controlled, randomized medication trial with a psychostimulant medication to assess the effects of medication on response variability. Advanced analytic methods utilizing non-Gaussian distributions and fast Fourier Transforms of the reaction time data will be used to conduct detailed analyses of RT patterns across the ADHD and normal control groups. Further, the effects of task parametric manipulations and medication on response variability will be examined. Finally, relations between response variability on neuropsychological tests and response variability in a variety of real-world analog situations will be examined to evaluate the ecological validity of these deficits.